In order to reduce the time of installation of electrical lines in ducts it is very important, to reduce friction between the electrical line and the inner surface of the cable duct.
From WO 02/39560 A1 it is known to apply a friction-reducing layer to the inner surface of the duct. The friction-reducing layer is a lubricating particulate solid phase, which includes carbon black and a lubricating plastic material.
WO 2004/114489 A1 discloses a tube for installation of flexible cables. The tube comprises an outer layer of low-density polyethylene and an inner layer of high-density polyethylene containing a blend of polyamide and polyether block amides with a silicone slip agent.
On the other hand it is known to reduce the friction by providing the outer surface of the outermost layer (the sheath) with longitudinal or helical ribs. Such a solution is described in DE 8901210 U1.
JP 01089210A proposes a cable to be pulled into a duct wherein a lubricant layer for reducing a friction coefficient of the cable and a rip cord for tearing an outer sheath are interposed between an inner sheath and the outer sheath. When tearing off the outer sheath with the rip cord the friction coefficient between the inner sheath and the duct line inner wall is lowered by the lubricant layer. Thus, the cable can be introduced smoothly into the duct line without adding surplus tension to it.
Still another solution of the described problem is object of JP 02123622. To improve the self-lubricating ability a resin film layer is obtained by adding a predetermined amount of a specific lubricant to the insulating layer on a conductor. For instance 0.2-5.0 weight part of alkylphenoxypoly (ethylene oxide) ethanole are added to 100 weight part of the insulating resin.
One main problem of this solution is the compatibily between the slipperiness and the coil packaging film. It is not possible to move the coil of the electrical line provided with the lubricant without rigid boxes. This is a main drawback for the producer of such electrical lines.